Proper wheel alignment and balance are essential in providing maximum safety, ease of handling, stability and directional control in all motor vehicles. The steering geometry of each vehicle must be adjusted to specifications recommended by the manufacturer. Different makes and models require different settings; these settings being published for each vehicle by each manufacturer, respectively. Wheel alignment and balance generally requires the following measurements, and corrections, to wit:
Set back: Set back refers to the relative forward positions of the two front wheels. When one front wheel is more rearward than another, this wheel is set back from the other.
Camber: Camber is the inward or outward tilt of a wheel at the top viewed from the front of the vehicle. Inward tilt is measured as negative camber, and outward tilt as positive camber. Camber measurements are taken from true vertical and measured in degrees. True vertical is zero.
Caster: Caster is the forward or backward tilt of the king pin or ball joint steering support arm at the top when viewed from the side. Forward tilt of the king pin or ball joint steering support arm is negative caster, and backward tilt positive caster. Caster is measured in the number of degrees from true vertical; true vertical being zero.
Steering axis inclination (or king pin inclination): Steering axis inclination is the inward tilt of the king pin or ball joint steering supports at the top. A line drawn through the centers of the ball joints is called the steering axis; this being the line about which the steering knuckle pivots as the wheel swings from left to right. Steering axis inclination is defined as the centerline around which the front wheel swings for steering. Steering axis inclination is measured in degrees.
The included angle: In front-end geometry, the included angle is the camber andle plus the steering axis inclination angle. This angle determines the point of intersection of the tire centerline with the steering axis or ball joint centerline. The angle is non adjustable, and the damaged parts must be replaced. Generally, the damaged part is a bent spindle, or bent spindle support arm.
Toe: Toe is the inward or outward tilt of a wheel at its front. Toe in of the front wheels occurs when the front wheels are closer together at the front than at the rear. Toe out occurs when the front wheels are further out in the front than at the rear.
Tracking: Tracking means that the rear wheels follow the front wheel in a parallel position. The rear wheels must follow the front wheels for good steering, handling and vehicle stability.
Vehicle safety depends to a large extent on a mechanic's ability to follow procedures and specifications recommended by the manufacturer on making wheel alignments, and adjustments. Camber, caster and toe in, for example, are adjustable on most vehicles, generally by the use of shims, eccentrics, slotted hole adjustments, or adjustable rods. Steering axis inclination and toe out on turns are not adjustable, but it is nonetheless essential that such measurements be made, and corrected. The precision and accuracy of prior devices for making such measurements varies greatly, and have many shortcomings.
The prior art teaches various combinations of apparatus for measuring the alignment of the steerable and non-steerable wheels of vehicles, e.g. caster, camber, toe, included angle, set back and track. The wheels are affixed to the body or chassis of a vehicle in manufacture as per engineering datum information to correlate the relationship of the wheels to the body or chassis of the vehicle. This datum information is commonly referred to as datum points and used as reference when repairing body and or chassis of wrecked vehicles. Since the non-steerable wheels of a vehicle are fixed by the manufacturer, and most are non-adjustable, special equipment is required to physically reposition these wheels. Most wheel alignment shops and service facilities do not have this type of equipment but it is nonetheless important that they be able to identify wheel to body, or chassis, relationships. The actual heading of the vehicle is usually determined by the thrust or track of the non-steerable wheels, which inturn affect the parallel relationship with the front wheels and their relationship with respect to steering wheel position.
For practical and economic considerations and for correcting so called mis-matching and tolerance buildup in manufacture, more and more manufacturers are making provisions for mechanically adjusting wheel positions relative to the body or chassis of the vehicle. This appears the best way for making corrections since the ideal geometric configuration of a four wheel vehicle is said to be a rectangle in which: (i) the steerable wheels will run parallel with each other and are equidistant from the center of the connecting axle or its equivalent; (ii) the non-steerable wheels will run parallel with each other and are equidistant from the center of the connecting axle or its equivalent; (iii) the non-steerable wheels either track with the steerable wheels or are equally off set from the steerable wheel tracks; and (iv) the vehicle body has it longitudinal geometric center line coincident with the longitudinal center line for both the steerable and non-steerable wheels. Unfortunately however, most prior art wheel alignment devices, and systems have not employed this ideal for making wheel alignment corrections. Most have relied upon interpreting some secondary references, or indicia, located adjacent the vehicle wheel, or chassis; information available only to the mechanic making the measurements. While alignment systems have been devised for remotely displaying like information, these alignment apparatus require excessive number of mechanical parts and electronic components. The required mechanical or optical coupling of apparatus of this type in sets to the steerable and non-steerable wheels complicate the procedure, which affect the accuracy of the information inasmuch as the procedure is usually entirely dependent upon the skill of the individual mechanic.